1. Field of the Invention
The present invention is directed generally to the field of magnetic tape data storage devices, and more particularly, but not by way of limitation, to an apparatus and method for reducing interference between data recorded on adjacent tracks on magnetic tape, thereby allowing an increase in the amount of data which can be stored on the tape.
2. Brief Description of the Prior Art
Magnetic tape data storage devices, also referred to as tape drives, have been used in the computer industry for years for the storage of large amounts of data. While magnetic disc drives, because of their greater speeds, have become the medium of choice for storing frequently accessed data such as application programs and user data which is being created or frequently modified, tape drives have achieved preeminence as storage devices for long-term and data backup purposes.
The technology of tape drives has evolved from large, expensive open reel machines to the current generations of cassette tape drives, which store large amounts of data in convenient self-contained cassettes. Historically, open reel tape drives recorded data on parallel data tracks which extend along the length of the tape, and utilized fixed data recording/retrieval heads, i.e., one dedicated read/write head for each data track.
The actual recording and recovery of data on the tape medium is accomplished by a gap in the read/write head, and is in the form of magnetic flux reversals formed in the magnetic coating on the tape. In order to maximize the sharpness of the flux reversals--and thus the amplitude of the read data pulses induced in the head during subsequent read operations--the length of the head gaps is aligned as precisely as possible with the direction of tape motion past the heads.
Historically, in order to ensure the integrity of data written on the tape, such tape drives included multi-gap heads, with one gap employed to write data and another gap, immediately trailing the write gap along the direction of tape motion, used as a read gap which could perform a read/verify operation on the data just recorded. If the tape drive was intended to record/recover data with the tape moving in both directions, an additional write or read gap was needed.
In tape drives of the type in which it is envisioned that the present invention would be employed, however, the head includes a single gap utilized for both writing and reading data, and data integrity is ensured through the use of ECC coding, or other data verification methods.
Several cassette-type tape drive formats have recently become industry standards, including the format referred to as the QIC, or quarter inch cassette, format in which the present invention is particularly useful. In QIC format tape drives, data are recorded on a plurality of data tracks which extend parallel with the length of the tape as was typical in open reel type tape drives, but employ only a single recording/playback head which is controllably movable to each of the data tracks. A commonly used mechanism for controlling the movement of the head from track to track employs a worm gear driven by a stepper motor, with the pitch of the worm gear and the radial precision of the stepper motor determining the accuracy of head movement, including the repeatability of multiple head movements to any one given track.
One of the major factors controlling the overall storage capacity of tape storage devices is referred to as track density, which is a definition, typically in data tracks per inch of tape width, of how closely the data tracks are spaced. The greater the track density, the greater number of tracks that can be recorded on a given width of tape and the greater the overall cassette data capacity.
A well known factor limiting track density is referred to as adjacent track interference, which is the corruption or loss of data brought about when data on a given track is written at a location touching or even overlapping the previously recorded data on an adjacent data track. In such a situation, the amplitude of the readback signal can be reduced, and there is a limit to the amount of readback signal reduction which can be tolerated and beyond which data can be corrupted or lost completely.
Another known factor controlling the ability of the tape drive to recover previously recorded data is a characteristic of the tape drive referred to as head azimuth, or simply azimuth, which is a measurement of the alignment between the longitudinal direction of the data tracks and the gap of the read/write head.
In the specifications defining the QIC tape drive and tape cassettes, one of the major planar surfaces of the cassette, called the cassette base plate, contains features which define a datum referred to as the tape cassette -B- plane. The tape cassette -B- plane is used, in conjuction with mating features on the tape drive which comprise a tape drive -B- plane, to define a mating surface between the tape cassette and the tape drive, and thus a base datum for defining the locations of both tape cassette and tape drive components and features along an axis normal to the common -B- plane.
Because the data tracks extend along the length of the tape, the length of the head gap which accomplishes the recording and retrieval of data on the data tracks is nominally parallel with the length of the data track and thus also nominally parallel to the -B- plane. It is known, however, that small deviations from this nominally parallel relationship are introduced by component and manufacturing tolerances. It is this geometric relationship between the length of the head gap and the -B- plane which is referred to as azimuth. When the length of the head gap is parallel to the -B- plane, or, in other words, when the width of the gap is perpendicular to the -B- plane, azimuth is considered to be zero, with deviations from parallel in a first direction being referred to as positive azimuth and deviations in the opposite direction being referred to as negative azimuth. Non-zero azimuths are typically measured in units of rotation, such as minutes.
In tape drives of the current generation, allowable deviations in azimuth typically include variations of, for example, .+-.10 minutes. When azimuth exceeds these allowable limits, signal degradation can lead to either data corruption or loss due to reduction in read signal amplitude. That is, if the azimuth of the gap during read operations is sufficiently tilted from the azimuth during a previous write operation, the magnetic flux reversals recorded will not be abrupt enough to induce read data pulses of a large enough amplitude to be distinguishable from noise inherent in the system.
Tape drives used for recording video images have made use of this knowledge for several years to reduce intertrack interference and maximize the amount of storage on a given area of tape surface. Most video recorders, however, utilize rotating, or helical scan, heads, rather than the linear heads toward which the present invention is directed, although some art does exist which is more relevant to the present invention.
U.S. Pat. Nos. 5,307,217 and 5,371,638, for instance, (hereinafter the '217 and '638 patents, respectively) disclose apparatus and methods directed to recording data at opposite azimuth angles on adjacent data tracks in order to minimize intertrack interference, and thus maximize data capacity on tape media. There are, however, several differences in both the type of tape drive in which the disclosed method and apparatus are employed and in the specific apparatus which implements the recording of data at opposite azimuth on adjacent data tracks.
Firstly, the '217 and '638 patents describe tape drives which include discreet tape reels which are mounted on individual drive motors, rather than the cassette tape drive in which the present invention is particularly useful.
Secondly, the '217 and '638 patents describe tape systems in which a read/verify operation is performed after every write operation, which necessitates the inclusion of specially fabricated heads, illustrated in both the '217 and '638 patents in FIGS. 2, 6 and 7. These custom heads include an arrangement of paired write gaps, laterally spaced and longitudinally aligned, with a read gap disposed laterally between the write gaps and offset from the write gaps along the longitudinal axis. With this arrangement of gaps, one of the write gaps and the single read gap are employed for accesses to one set of alternate tracks, while the other write gap and the read gap are used to record and recover data on the interleaved other set of alternate tracks. Both the '217 and '638 patents note that the same results could be achieved using a pair of laterally spaced read gaps and an intermediate, offset write gap. This disclosure contrasts sharply with the single write/read gap head envisioned for use in the tape drive of the present invention.
Thirdly, the '217 and '638 patents disclose an apparatus for rotating the head to align selected write gap/read gap pairs with the desired data track which includes mounting the head on the shaft of a rotary motor and mounting the rotary motor/head assembly on a stepper motor-driven linear actuator for movement from track to track. This mechanism is very different from the apparatus of the present invention, as will be apparent from the description to follow.
A need clearly exists for a method and apparatus for minimizing or eliminating adjacent track interference, thus leading to increased data reliability, increased track density and increased overall data capacity in tape drives.